Generally, when two nuclear magnetizations having different resonance frequencies interact, J interaction due to electrons and NOE (nuclear Overhauser effect) interaction due to nuclear dipoles take place. Nuclear Overhauser effects are divided into longitudinal NOE and transverse NOE. The longitudinal NOE occurs in the direction of the static magnetic field, while transverse NOE is observed in a direction perpendicular to the direction of the magnetic field. Since an r.f. field is normally applied perpendicular to the static magnetic field, the transverse NOE takes place within the same plane as the r.f. field, the transverse NOE takes place within the same plane as the r.f. field. Either J interaction or an NOE interaction can be induced according to the experiment method. If the interaction is known, internuclear distances can be measured. This enables analysis of a molecular structure. The transverse NOE is also called ROESY (rotating-frame Overhauser enhancement spectroscopy). This spectroscopy is described in detail in an article entitled "Practical Aspects of Two-Dimensional Transverse NOE Spectroscopy" by Ad Bax and Donald G. Davis in Journal of Magnetic Resonance, 63, pp. 207-213 (1985).
In the prior art rotating-frame Overhauser enhancement spectroscopy (ROSEY) of one- or two-dimensional high-resolution NOE spectroscopy, during interaction due to NOE after the creation of transverse magnetization vertical to the static magnetic field, an r.f. field of a given strength is applied transversely to spin-lock the magnetization.
FIG. 1 shows a measuring method using spin-locking in one-dimensional NMR spectroscopy. First, a 90.degree. pulse is applied to create transverse magnetization. Then, an r.f. field is applied in the direction of the transverse magnetization to spin-lock the magnetization. Thus, the directions of the magnetizations are aligned to cause interaction. After a lapse of a given period, the spin-locking is discontinued to permit the magnetization precessing about the axis of the static magnetic field to decay freely. The resulting free induction decay is detected to observe the interaction of magnetization.
Referring next to FIG. 2, in two-dimensional NMR spectroscopy, transverse magnetization is created by a 90.degree. pulse. After a lapse of evolution period t.sub.1, an r.f. field is applied to spin-lock the magnetization. After stopping the spin-locking, the resulting free induction decay is observed in the same way as in the aforementioned one-dimensional NMR spectroscopy. This technique is disclosed, for example, in the above-cited Journal of Magnetic Resonance, 63, pp. 207-213 (1985).
Where two spin systems interact with each other, such spin-locking alters the magnitudes of spin. The changes in the magnitudes are detected, whereby the interaction can be observed.
We now discuss one-dimensional NMR spectroscopy. As shown in FIG. 3, a 90.degree. pulse is applied in the y-direction. It is assumed that the magnetization is tilted and becomes oriented in the x-direction by the application of the 90.degree. pulse. The magnetization tilted by 90.degree. is about to rotate within the xy-plane about the axis of the static magnetic field, but the magnetization is locked in the x-direction, because an r.f. field is immediately applied in the x-direction. However, for magnetization having a different resonance frequency, the apparent r.f. magnetic field is the vector sum of the x-direction r.f. magnetic field and the vertical component of the field because of the vertical component magnetic field which depends on the difference between the frequency of the magnetization and the resonance frequency, or offset. The vector sum is represented by RF' in FIG. 3 and tilted to the z-axis by angle .theta.. Let MX.sub.0 be off-resonance magnetization oriented in the x-direction. Since the angle formed between RF and RF' is .theta., what is spin-locked by the off-resonance magnetization is MX.sub.0 cos .theta.. The vertical component of the spin-locked off-resonance magnetization (in the direction of RF') rotates at a high speed within a plane perpendicular to the r.f. magnetic field. Because the r.f. field is not completely homogeneous, the vertical component magnetization has inhomogeneous phases and so it cannot be observed. After discontinuing the spin-locking, interaction due to the component oriented in the same direction as RF, i.e., MX.sub.0 cos.sup.2 .theta., is observed.
In this way, in the prior art NOE spectroscopy, the interaction is observed in the form of the intrinsic magnetization MX.sub.0 multiplied by cos.sup.2 .theta.. As a result, the sensitivity deteriorates.
In order to avoid this deterioration, an increase in the strength of the r.f. field may be contemplated. However, if it is increased excessively, J interaction also appears other than the transverse NOE. Also, this places great burden on the instrumentation. Further, longitudinal NOE is observed, as well as transverse NOE. For these reasons, it has been impossible to increase the r.f. field sufficiently.
A method of preventing such sensitivity deterioration in two-dimensional NMR spectroscopy has been recently proposed. This method entails phasing of spectra. Especially, in one-dimensional NMR spectroscopy, sensitivity deterioration cannot be avoided.